Electroplating plastics

ABSTRACT

ELECTROPLATING OF A MOLDABLE PLASTIC, WITH OR WITHOUT AN INERT FILLER THEREIN, IS ACHIEVED BY CONDITIONING A PREFORMED ARTICLE OF SAID PLASTIC BY A COMBINATION TREATMENT CONSISTING OF AN ACID CHROMATE EACH AND A PERSULFATE TREATMENT. THE RESULTING CONDITIONED ARTICLE IS THEREAFTER PLATED BY PREPLATING WITH AN ELECTROLESSLY PLATABLE METAL FOLLOWED BY ELECTROPLATING THE PREPLATED ARTICLE WITH A FINAL FINISH TO OBTAIN A METAL-PLATED PLASTIC PRODUCT.

United States Patent 3,597,336 lELECTROPLATING PLASTICS James A. Shotton, Bartlesville, Okla, and Jimmy S. Dew, Texas City, Tex., assignors to Phillips Petroleum Company No Drawing. Filed Apr. 22, 1968, Ser. No. 723,262

rm. c1. C23b /64 US. or. 204-30 Claims ABSTRACT OF THE DISCLOSURE Electroplating of a moldable plastic, with or Without an inert filler therein, is achieved by conditioning a preformed article of said plastic by a combination treatment consisting of an acid chromate etch and a persulfate treatment. The resulting conditioned article is thereafter plated by preplating with an electrolessly platable metal followed by electroplating the preplated article with a final finish to obtain a metal-plated plastic product.

This invention relates to the electroplating of plastics. In one aspect, this invention relates to an improved process for electroplating plastics. In another aspect, this invention relates to a method for providing electroplated plastic articles having an improved adhesion of the metal plate thereto. In a further aspect, there is provided a method for conditioning plastic articles for use in an electroplating process.

BACKGROUND OF THE INVENTION The market for electroplated-plastic articles has expanded tremendously in the last few years. By electroplating plastics the inherent advantages of plastics, i.e., economy, light weight, corrosion resistance, moldability, etc., become available to both fabricators and users of electroplated parts. Present applications for electroplated plastics include automotive accessories (e.g., knobs, handles, trim, bezels), appliance parts (e.g., housings, grills, handles), plumbing fixtures (e.g., valve bodies, sink strainers, P-traps, showerheads), houseware and furniture parts (e.g., knife handles, soap dishes, lamp bases, picture and mirror frames) and industrial uses where the characteristics of its abrasion resistance and shielding qualities are desirable.

Although the demand for electroplated plastics has expanded, not all plastics are capable of being plated by conventional processes due to the failure to obtain a satisfactory adhesion of the metal plate to the plastic Thus, for example, olefin polymers have found little utility in the formation of plated plastic articles because it has not been possible to obtain a strong bonding of the metal plate to the polymer surface. This difiiculty is apparently due to the physical and chemical properties of the olefin polymers such as polyethylene, polypropylene, halogenated olefin polymers and the like which render them inert and, accordingly, unaffected by conventional etching techniques normally employed in electroplating.

Heretofore the most widely used plastic for electroplating was acrylonitrile-butadiene-styrene, hereinafter referred to as ABS, since conventional plating techniques can be employed with this plastic and the plating bonds Well. For other plastics than ABS the poor bonding of the plate, if formed by conventional techniques, resulted in a limited use of the resulting product. The necessity of having to use other than conventional plating techniques for electroplating plastics not only materially increases the cost of the electroplated product but till now has been unable to overcome the problem of poor bonding or adhesion of the metal plate so that use of such plated products has been limited to small parts such as knobs.

For these reasons, as well as the outstanding properties possessed by other plastics and, particularly, the olefin polymers, there has been a continuing effort to develop a method for electroplating other plastics by conventional techniques.

Accordingly, it is an object of this invention to provide an improved process for metal plating plastics.

Another object of this invention is to provide a method for producing a plated plastic article having improved adhesion of the plate to the plastic surface.

A further object of this invention is to provide a process for electroplating plastics which permits the utiliza tion of conventional electroplating systems.

Another object of this invention is to provide a method for conditioning plastics so as to provide a product suitable for use in conventional electroplating systems.

Other aspects, objects and several advantages of this invention will be apparent to those skilled in the art from the following description and appended claims.

THE 'INVENTION According to this invention there is provided a process for electroplating a plastic which comprises the steps of (l) molding a plastic composition (with or Without filler therein) into a desired configuration;

(2) conditioning the resulting molded plastic product by treating, in series, with (a) an acid chromate etch and (b) a persulfate treatment;

(3) preplating the conditioned article with an electrolessly platable metal; and

(4) electroplating the preplated article with a final finish .to obtain a metal plated plastic product.

Although the overall process for electroplating plastics in accordance with this invention utilized the aforementioned steps, it is not essential that all the steps be performed-as a single operation.

Accordingly, once conditioned by the persulfate treatment step of this invention, either alone or followed by an acid chromate etch step, the surface conditioned molded plastic article can be immediately preplated or can be shipped to some other location for further processing. Thus, the persulfate treatment step results in the formation of a novel product suitable for further processing in conventional electroplating systems.

Likewise, while each step of the conditioning treatment of this invention is conventionally carried out immediately in sequence, in one embodiment of this invention wherein the persulfate treatment is utilized following the acid chromate etch treatment, the persulfate-treated plastic article resulting therefrom represents a novel surface conditioned article which can be supplied without further treatment to operators of conventional plating processes. This is advantageous in some instances since it permits conventional plating processors to form plated plastic articles without having to alter their established operations or by having to acquire additional equipment for forming the persulfate-treated article.

Also, the electrolessly plated plastic article prepared by the above steps, 1 through 3, represents a novel preplated product which, if desired, can be supplied in this form to a processor for electroplating. This is particularly advantageous when the final finish metal on the plastic anticle is to be of a type not normally utilized and the preparation of which by the preplator would be uneconomical.

Although the process of this invention can be utilized for the plating of any moldable plastic as defined above, in one specific embodiment of this invention it has been found to be particularly suitable for electroplating olefin polymers.

The term olefin polymer as used herein and in the claims is intended to include both homopolymers and copolymers or mixtures thereof of aliphatic l-olefins having 2 to 8 carbon atoms.

The process of this invention can be utilized with any moldable plastic material. As used herein and in the claims the term plastic is intended to include any natural or synthetic polymeric material which can be molded into a desired final shape using heat and/ or pressure.

Examples of such materials, which are listed and described in Modern Plastics Encyclopedia for 1967, are: ABS resins, acetal resins, acrylics and modified acrylics, alkyd resins, allyl resins, amino resins, halogenated polyethers, epoxy resins, fluoroplastics, furane resins, ionomers, isocyanates, nylons, parylene polymers, phenolics, phenoxy resins, polyalkenes, polycarbonates, polyesters, polyimides, polyarylene oxides, polyarylene sulfides, polysulfones, silicones, styrene polymers and copolymers such as styrene/butadiene, vinyl polymers and copolymers, such as poly(vinyl chloride), poly(vinyl fluoride), vinylidene chloride/vinyl chloride copolymer, and the like, including blends.

The terms mold, molded, moldable, molding, and the like as used herein and in the claims is intended to include any plastic forming process such as film formation by extrusion, casting, or calendering, blow molding, injection molding, extrusion, vacuum forming, pressure forming, compression molding, transfer molding, casting, thermoforming, and the like.

Examples of the aliphatic l-olefins are ethylene, propylene, l-butene, l-pentene, Z-methylpentene, l-hexene, l-heptene, l-octene, 1,3-butadiene, 1,4-hexadiene, 1,3- heptadiene, 1,5-octadiene, and the like. Examples of the olefin polymers and polymer blends that can be used are polyethylene, polypropylene, poly(butene-l), poly- (Z-methylpentene), ethylene/propylene copolymer, ethylene/butene-l copolymer, ethylene/hexene-l copolymer, ethylene/propylene/1,4hexadiene terpolymer, ethylene/ propylene/butene 1 terpolymer, polyethylene/polypropylene blends, polyethylene/polypropylene/polydiene blends and the like.

Such polymers are known to be inert to most chemicals and when heretofore plated formed products having poor metal to plastic adhesion values. This, in turn, limited the end uses of such metal plated items to those applications where the strength of the bond or adhesion value was not critical.

When carrying out the process of the present invention with an olefin polymer, there is achieved a substantial increase in the metal to plastic adhesion values of the plated article.

The surface conditioning step of the present invention utilizes a combination, in series, of (a) an acid chromate etch, and (b) a persulfate treatment. The specific sequence of these two operations is not critical. Accordingly, the plastic article can be treated with the acid chromate etch followed by the persulfate treatment or, conversely, by the persulfate treatment followed by the acid chromate etch. To avoid contamination of the various treating agents the plastic article should be rinsed after each separate treatment.

The term persulfate treatment as used herein and in the claims is intended to cover treatment of the molded plastic article with an aqueous bath containing persulfuric acid and/or ammonium persulfate and/or one or more Group I metal persulfates in a concentration range of 25 weight percent to saturation. The term persulfate as used herein and in the claims is meant to embrace the terms peroxysulfate and peroxydisulfate.

Persulfuric acid can be formed in situ by electrolysis of sulfuric acid at temperatures below about 85 F. using smooth platinum electrodes and a high current density. The Group I metal persulfates can be formed in the same manner, but it is usually more convenient to prepare the treating bath from commercially available persulfates, such as sodium persulfate, potassium persulfate, ammonium persulfate, and the like.

It is within the scope of the invention also to have sulfuric acid present in the persulfate bath. When sulfuric acid is present, the mole ratio of sulfuric acid to persulfuric acid and/or persulfate is in the range of 0.5/1 to 2/1.

The persulfate treatment is ordinarily carried out for a period of 0.1 to 10 minutes, and preferably 2 to 4 minutes.

The persulfate treatment is normally carried out at room temperature but can be carried out at temperatures up to essentially the boiling point of the solution.

Temperatures above the normal boiling point can be used provided the pressure is sufficient to maintain a liquid phase. Temperatures in the range of 30 to 200 F. are suitable.

The acid chromate etch treatment is that conventionally employed in metal plating and is normally carried out at temperatures in the range of 75 F. to 200 F. The acid chromate etch treatment is generally carried out for a period of time in the range of 0.1 to 20 minutes. However, when used in the treating of olefin polymers in accordance with this invention, a treating time of 2 to 4 minutes is preferred.

The acid chromate etch solution is usually formed by the addition of a chromate 'salt or CrO to a strong acid such as sulfuric acid or a sulfuric acid-phosphoric acid mixture. Typical of such a solution is that obtained by mixing the following:

(A) Concentrated H 50 (238 ml.) saturated with (B) H O(262 ml.) saturated with CrO at F., and (C) H O to make a total of 1100 ml.

The presence of fillers in the plastic is not essential to the achieving of the satisfactory plating of the plastic article when utilizing the process of this invention.

However, the process can be satisfactorily carried out utilizing a moldable plastic having a filler therein.

Fillers, when present, are generally in an amount of 1 to 35 weight percent based upon the total weight of filled plastic and filler. All percentages recited herein are on this basis. By filled plastic is meant that portion of a plastic molding into which the filler has been incorporated. The filler or mixtures thereof can be incorporated into the plastic by any known technique using known milling and blending equipment, such as'a Banbury mixer, a Brabender Plastograph, and the like. The filler can be distributed throughout the plastic stock or, when the plastic is in sheet or film form, can be present only in the surface layer of the plastic film or sheet, optionally in one or both surfaces.

Examples of fillers that can be optionally included in the plastic composition prior to molding are the metal oxides, such as titania, magnesia, calcia, silica, and the like, or the naturally-occurring minerals having a high-- i.e., greater than about 70 weight percentcontent of metal oxide, such as diatomaceous earth, talc, pumice, anatase, brookite, brucite, periclase, rutile, and the like, including mixtures thereof. The inorganic filler is generally utilized in a form having a particle size less than 50 microns, and preferably less than 10 microns. The fillers used in the examples of this application all had a particle size less than 50 microns.

As indicated, it would be highly desirable in the electroplating of a plastic article to be able to utilize conventional plating processes in producing a plated product having good adhesion of the metal plate thereto.

Such conventional plating processes involve a preplating process which includes cleaning; conditioning or etching the surface of the plastic with an acid chromate solution, such as chromic-sulfuric acid, at elevated temperatures; sensitizing the surface of the plastic with an oxidizable salt, such as stannous chloride, that is absorbed and later reduces the activator (not all conventional processes include this step); activating the surface with a precious metal salt, such as palladium chloride; and electroless plating with either copper (about 0.005 mil to 0.010 mil) or nickel (about 0.010 to 0.030 mil). Each conditioning step is followed by one or more water rinses. The continuous film of electrically conductive material applied by the preplating process provides the capability for applying the final finish by conventional electrolytic processes. Following the preplate process, normal plating of copper-nickelchrome, or nickel-chrome or any of a whole variety of final finishes, including gold and silver, can be applied by conventional electroplating techniques. For most applications the final plate will be about 0.5 to 2.0 mils thick, but even thicker plate can be applied if desired.

The following procedures are representative of the con- =ventional plating processes and conditions which can be used in the electroplating of plastics in accordance with this invention.

It is to be understood that the recitation of specific plating solutions and steps in no way limits the invention to these specific solutions and steps. There are numerous plating systems available, and the process of the invention can be used with any of them, so long as a persulfate treatment, as herein defined, precedes or follows the acid chromate etch.

(1) Immerse in a sodium pyrophosphate cleaning solution for 2 to minutes at 140 F.

(2) Immerse in a sodium bisulfate neutralizing solution for 15 to 30 seconds at 75 F.

(3) Immerse in an acid chromate etching solution for 0.1 to 20 minutes at 75 to 200 F.

(4) Rinse with 5 weight percent hydrochloric acid.

(5) Immerse in a stannous chloride sensitizing solution for 15 to 60 seconds at 75 F.

(6) Immerse in a palladium ammonium chloride activating solution for 15 to 60 seconds at 75 F.

(7a) Immerse in an electroless copper plating solution for 5 to 30 minutes at 75 F. The plating solution comprised modified Fehling solutions: solution A was CuSO and solution B was NaOH, NaK tartrate, Na CO and NalczHa oz, 01'

(7b) Immerse in an electroless nickel plating solution for 5 to 30 minutes at 75 F. The plating solution usually contains nickel salts and a reducing agent such as sodium hypophosphite or a boron amine.

(8a) Strike with copper. The composition of the copper strike bath and conditions for plating were as follows:

Composition of the copper strike bath 98 grams-CuSO -5H O 15.5 mil1iliters-concentrated H 50 1 milliliterUBAC Brightener No. 1 Suificient water to make 1 liter of solution Supplied by Udylite lCorporation, Detroit, Mich.

Plating conditions VoltageZ volts D.C.

Current density10 to amperes/ft. Current efficiency-400% Anodeelectrolytic copper Temperature75 to 80 F.

Time4 to 10 minutes Agitated bath, or

(8b) Strike with nickel. The composition of the nickel strike bath and conditions for plating were as follows:

Composition of the nickel strike bath 300 410 grams-NiSO 6H O 30-45 gramsNiCl 3 /2H O 10 ml. nickel brightener N E 2 Sufiicient water to make 1 liter of solution Supplied by Udylite Corporation, Detroit, Mich.

Plating conditions Voltage6-18 volts D.C.

Current density--30'-8O amperes/ft. Current etficiency-100% Anodenickel (99.5% Temperature-7 5155 F. Time4-10 minutes Agitated bath Composition of the bright copper bath 212 grams--CuSO -5H O 28.8 millilitersconcentrated H 4 milliliters-UBAC Brightener No. l 75 milligramsNaCl Sufiicient water to make 1 liter of solution Plating conditions Voltage4 volts D.C.

Current density30 to 40 amperes/ft. Current efliciency98 to 100% Anode electrolytic copper Temperature-75 to 80 F.

Timel to 3 minutes 3 Agitated bath 3 45 minutes used in preparing the test specimens.

(B) Electroplate with nickel. The composition of the nickel plating bath and conditions for plating were as follows:

Composition of the nickel plating bath 1 13 6 gramsNiSO 6H O 312 grams-NiCl 185 gramsH BO Sufiicient water to make 1 gallon of solution Plating conditions Voltage-4 volts D.C.

Current density-40 to 50 amperes/ft. Current efficiency to Anodenickel Temperature-75 to 160 F.

Time-3 0 to seconds Agitated bath (C) Electroplate with chromium. The composition of the chrome plating bath and the conditions for plating were as follows:

Composition of the chrome plating bath 3 5 0 gramsCrO 2 millilitersconcentrated H 50 Sufficient water to make 1 liter of solution Plating conditions Voltage6 to 8 volts D.C.

Current density-90 to 110 amperes/ft.

Current etficiency-20% Anode-lead Temperature-80 to F.

Time-30 to 90 seconds Agitation of the bath effected by the evolution of gases Steps 1 and 2 of the conventional plating process form a cleaning operation to remove any dirt or other foreign matter from the surface of the preformed or molded object to be plated.

Step 4 of the above-described plating process is not required if the persulfate treatment of the conditioning step of this invention is carried out after the acid chromate etch, and was not used in the tests made to illustrate the process of the invention. When the persulfate treatment is carried out prior to the acid chromate etch, in the conditioning step of the process of the invention, then step 4 of the conventional electroplating process can be utilized.

When the surface conditioned molded plastic article is to be immediately plated in the conventional plating process, steps 1 and 2 of the conventional plating process as described above are not required. Thus, following the conditioning treatment utilizing the combination of persulfate treatment and acid chromate etch, the resulting plastic article is sensitized with an oxidizable salt (5) followed by the remaining steps as described for electroless plating and electroplating.

Ordinarily each conditioning and plating step is followed by one or more Water rinses.

The following specific examples are presented to further illustrate the invention but should not be interpreted to restrict or limit the invention. The plastic-filler blends used in these examples were prepared by blending the indicated amounts of plastic and filler in either a Brabender Plastograph for 5 to minutes at 50 75 r.p.rn. and about 370 F. in a nitrogen atmosphere or in a Banbury mixer for about 5 minutes at about 350 F. in an air atmosphere. Compression-molded or injection-molded slabs having a thickness of 50 to 75 mils were prepared from the blends and 3 /2 inch by 1 /2 inch pieces were cut from the molded slabs for the plating tests.

Adhesion values were determined in an adhesion test made by pulling the metal layer from the plastic or filled plastic in an Instron tester at a 90 angle and at a rate of 2 inches per minute. In this test a steel bar /2-inch wide is laid down the center of the 3% inch by 1 /2 inch piece of plated plastic and a sharp knife is used to cut through the electroplate along each side of the bar. One end of the resulting /2-inch wide strip is pulled loose for /2 to /1 inch. A clamp attached to a wire about 2 feet long is attached to this loosened metal tab. The plastic or filled plastic is attached to the traverse in the Instron tester and the wire to the upper jaw. The long wire is used so that the angle does not change appreciably as the metal is pulled at right angle from the plastic surface. The average value of the force, in pounds, required to separate the metal and plastic is multiplied by two to get the force required per lineal inch of contact. In the specimens prepared for this test the bright copper electroplate was 2 to 2.5 mils thick so that the metal itself would not yield during the test, and the nickel and chromium electroplating steps were not used.

EXAMPLE I A 96 weight percent propylene-4 weight percent ethylene copolymer having a melt flow of 1.5 to 2 dg./ min. (ASTM D 123862T, Condition L) and a density of 0.899 g./cc. (ASTM D150563T) was blended with weight percent finely divided silica. Adhesion values obtained using either an acid chromate etch bath (5 minutes a=t1 60 F.) or both a 50 weight percent ammonium persulfate bath (5 minutes at 80 F.) plus an acid chromate bath (5 minutes at 160 F.) are:

Etch bath: Adhesion, lb./ in. Acid chromate 2.5 Persulfate+acid chromate 4.5

Samples containing different percentages of filler were also tested with a 40-minute etch in acid chromate (160 F.) and with a 20-minute etch in 50 weight percent ammonium persulfate F.) followed by a 20-minute etch in acid chromate (170 F.):

Adhesion, lb./in.

Single Double Filler; wt. percent bath etch bath etch 1 No plate. I 2 The persulfate bath was a mixture of 36 weight percent ammonium 5 persulfate and 15 weight percent potassium persulfate, and was used at EXAMPLE II Polypropylene having a melt flow of about 4 dg./min. and a density of 0.905 g./cc. was blended with various amounts of various finely divided fillers. Adhesion values using an acid chromate bath followed by a 50 weight percent ammonium persulfate bath are:

Time, min/temp, F.

Fillenwt. Acid cln'o- Persnlfate Adhesion,

percent mate bath bath lb./iu.

None 20/160 20/80 0. o T102; 10 20/160 20/80 2.8 T102; 20 20/160 20/80 5. 0 Talc; 10 20/160 20/80 4. 8 A1203; 10 20/190 20/80 8.0 S102; 10.- 20/160 20/80 7.0 ZnO; l0 20/170 20/80 5.0+ MgO; 20 1 10/160 10/s0 s. 0

1 1n ethylene-propylene copolymer of Example I.

EXAMPLE III Tests were made in the same manner as in Example II, using different amounts of finely divided silica filler in polypropylene and using the persulfate bath first (20 minutes at 80 IR), followed by the acid chromate bath (20 minutes at F.):

SiO wt. percent: Adhesion, lb./in.

EXAMPLE IV Tests were made in the same manner as in Example III, using 5 weight percent silica filler, and using a persulfate bath containing 35 weight percent ammonium persulfate and 15 weight percent potassium persulfate (20 minutes at 130 F.) followed by an acid chromate bath (20 minutes at F.):

SiO wt. percent: Adhesion, lb./ in. 0 3.2

EXAMPLE V Adhesion, 1b./in.

From the foregoing examples it can be seen that utilization of the combination treatment consisting of an acid chromate etch and a persulfate treatment for the conditioning of a moldable plastic article with or Without a filler therein results in the obtaining of a metal plated product having improved adhesion.

In addition, the examples demonstrate that utilization of the conditioning treatment of the present invention permits the satisfactory plating of a plastic article in conventional metal plating processes.

Reasonable variations and modification of this invention can be made or followed in view of the foregoing disclosure, without departing from the spirit or scope thereof.

We claim: 1. A process for electroplating a moldable plastic which comprises the steps of (1) molding said plastic into a desired configuration; (2) conditioning the molded plastic product of step (1) by contacting the molded plastic product with a combination treatment, in series, which comprises (a) an acid chromate etch bath obtained by mixing H O, concentrated H 80 saturated with K Cr O at 80 F. and H saturated with CrO at 80 F. and (b) an aqueous persulfate bath containing therein in a concentration in the range of weight percent to saturation at least one of the group consisting of persulfuric acid, ammonium persulfate and Group I metal per-sulfates; (3) preplating the resulting conditioned product of step (2) with an electrolessly platable metal; and

(4) electroplating the preplated product of step (3) with a final finish to obtain a metal-plated plastic product.

2. A process according to claim 1 wherein the molded product of step (1) is washed prior to conditioning with the oxidation treatment of step (2).

3. A process according to claim 1 wherein the molded product of step (1) is conditioned by being contacted first with an acid chromate etch bath (a) and then with the persulfate bath (b).

4. A process according to claim 1 wherein the molded product of step (1) is conditioned by being contacted first with the persulfate bath (b) and then with the acid chromate etch bath (a).

5. A process according to claim 1 wherein said moldable plastic contains from O to 35 weight percent filler.

6. A process according to claim 1 wherein said moldable plastic is an olefin polymer.

7. The product of the process of claim 1.

8. A process for conditioning a molded plastic product having 0 to 35 weight percent of a filler therein so as to render same susceptible to electroplating which comprises contacting said molded plastic product with a combination treatment, in series, which comprises (a) an acid chromate etch bath obtained by mixing H O, concentrated H saturated with K Cr O- at 80 F. and H 0 saturated with CrO at 80 F. and (b) an aqueous persulfate bath containing therein in a concentration in the range of 25 weight percent to saturation at least one of the group consisting of persulfuric acid, ammonium persulfate and Group I metal persulfates.

9. The conditioned molded plastic product of the process of claim 8.

10. A process according to claim 8 wherein said plastic is an olefin polymer.

References tlited UNITED STATES PATENTS 2,447,379 8/1948 Wenger 11747 3,142,582 7/1964 Koretzky et a1 11747 3,157,527 ll/ 1964 Fournet et a1 11747 3,219,557 11/1965 Quintana 20420 3,434,867 3/1969 Rousselot 11747 3,436,233 4/1969 Jackson 1174RX HOWARD S. WILLIAMS, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. Cl. XJR.

r 204 -20; 11747R c.) 

